Cascaded pump delivery for remotely pumped erbium-doped fiber amplifiers

ABSTRACT

A method for pumping remote optically-pumped fiber amplifiers (ROPAs) in fiber-optic telecommunication systems is disclosed which uses cascaded Raman amplification to increase the maximum amount of pump power that can be delivered to the ROPA. According to the prior art, high power at the ROPA pump wavelength, λp, is launched directly into the fiber and the maximum launch power is limited by the onset of pump depletion by Raman noise and oscillations due to the high Raman gain at ˜(λp+100) nm. In preferred embodiments of the present invention, a ‘primary’ pump source of wavelength shorter than λp is launched into the delivery fiber along with two or more significantly lower-power ‘seed’ sources, among which is included one at λp. The wavelength and power of the seed source(s) are chosen such that, when combined with the high-power primary source, a series, n, where n≧2, of Raman conversions within the fiber ultimately leads to the development of high power at λp. In another embodiment, one or more of the seed sources at wavelengths shorter than λp are replaced by reflecting means to return, into the fiber, backward-travelling amplified spontaneous Raman scattered light resulting from high power in the fiber at a wavelength one Raman shift below the particular seed wavelength. In either case, the high power at λp is developed over a distributed length of the fiber, reaching its maximum some distance into the fiber and exceeding the maximum power possible at that point with the prior art.

FIELD OF THE INVENTION

The present invention relates to amplification in optical fibertelecommunication spans and, more particularly, to remoteoptically-pumped erbium-doped fiber amplifiers, as are used in submarinefiber optic spans and long distance spans over land.

BACKGROUND OF THE INVENTION

The amplification of optical signals in fiber-optic telecommunicationsystems is achieved primarily through the use of discrete opticalamplifiers, mainly erbium-doped fiber amplifiers (EDFAs), and throughdistributed Raman amplification (DRA) in which the transmission fiberitself is used as the amplifying medium.

Discrete amplifiers placed as boosters or pre-amplifiers at either endof an optical cable link are sufficient for moderate span lengths andcapacities. However, as capacity and/or span length requirementsincrease in unrepeatered systems, distributed Raman amplification istypically implemented first and then, when even this is not sufficient,architectures with discrete EDFA amplifiers placed out in the cable andpumped remotely from the terminals are used. According to the prior art,these remote optically pumped amplifiers (ROPAs) are pumped by launchinghigh power at 1480 nm from either the receiving terminal of the link inthe case of a remote preamplifier or the transmitting terminal for aremote post amplifier. In the latter case, the 1480-nm power isdelivered through one or more dedicated pump fiber(s) to avoid anyinteractions between the launched signal channels and the pump. A remotepreamplifier can be pumped via the transmission fiber itself or adedicated pump fiber or both.

The increase in link budget achievable through the addition ofremotely-pumped amplifiers is determined by the maximum distance overwhich the required ROPA pump power can be delivered. Increasing theamount of pump power launched increases this distance, up to a point. Asthe launched 1480-nm pump power is increased, the resulting high Ramangain in the 1590-nm region begins to deplete the 1480-nm power deliveredto the ROPA via the build-up of Raman noise and eventually, oscillationsaround 1590 nm. As pointed out by Boubal et al., SubOptic'2001, Kyoto,paper P3.6 (May 2001), the maximum 1480-nm launch power in standard puresilica core fiber (PSCF) for example, is ˜1.3 W. In other words, for theeffective transmission of the 1480-nm pump energy to the ROPA, the Ramanproperties of optical fiber place an upper limit on the maximum 1480-nmlaunch power. This limit can be increased to ˜1.9 W by utilizing asegment of Large Effective Area PSCF (E-PSCF) leading away from the1480-nm launch terminal. See for instance, E. Brandon et al.,SubOptic'2001, Kyoto, paper T3.4.1 (May, 2001).

A further increase to ˜4 W has been demonstrated by incorporating fusedWDM couplers in dedicated pump fibers to act as filters with low loss at1480 nm but high loss in the region of high Raman gain. See forinstance, Boubal et al., SubOptic'2001, Kyoto, paper P3.6 (May 2001). Ina recent development, this same group combined the use of a hybridPSCF/E-PSCF dedicated pump fiber incorporating fused WDM coupler filterswith a first-order Raman pumping scheme to further increase the distanceover which the required pump power could be delivered to a remotepreamplifier. See L. Labrunie et al., Electronics Letters, Vol. 39, No.19 (September, 2003). In this scheme, they launched high power at 1387nm along with substantially lower power at 1480 nm from a laser diode.The high power at 1387 nm provided Raman gain for the 1480-nm power asit propagated along the dedicated pump fiber and resulted in a netincrease in the 1480-nm power reaching the ROPA compared to aconventional pumping scheme involving the direct launch of high power at1480 nm alone.

Despite the improvements in delivered pump power provided by theselatter developments, they require dedicated pump fibers even for aremotely-pumped preamplifier, a fact which, in and of itself, hassignificant cost implications. Furthermore, the dedicated pump fiber isa hybrid PSCF/E-PSCF fiber incorporating WDM coupler filters, which addsto the complexity and cost of the pump fiber. In today's cost-sensitiveenvironment, there is an ever-present need for performance improvementswhich have the least possible negative impact on the cost of fiber-opticcommunications systems. Thus, a ROPA pumping scheme, such as thatdisclosed in this application, which increases the pump powerdeliverable to remote amplifiers and, especially in the case of remotepreamplifiers, does so with minimal impact on cost, is highly desirable.

SUMMARY OF THE INVENTION

In a broad aspect, the invention provides a scheme for increasing theamount of pump power that can be delivered to a remote optically pumpedamplifier in an optical fiber communication system or, conversely,increasing the distance along the transmission span over which a givenamount of pump power can be delivered. According to this scheme,high-order Raman amplification is used to provide amplification in thetransmission span itself, or in a dedicated pump fiber, for therelatively low power at the 1480-nm ROPA pump wavelength launched fromthe receiving or transmitting terminal, for a remote preamplifier orpost amplifier, respectively. This pumping scheme can, in acost-effective manner, significantly improve the pump power deliverycompared to prior-art pump delivery schemes.

More specifically, in a typical embodiment, a primary pump source at apredetermined wavelength λ₀, shorter than the ultimately desired ROPApump wavelength λ_(p), is launched into the pump delivery fiber (whichmay be either the transmission fiber itself or a dedicated pump fiber)along with two or more lower-power seed sources at wavelengths λ_(s1) .. . λ_(sn), where n≧2 and λ₀<λ_(sn)≦λ_(p), and where the ensemble ofseed sources contains at least one at the ROPA pump wavelength λ_(p).The wavelength and power of the seed sources are specifically chosensuch that, in the presence of the pump power at λ₀, a series of nstimulated Raman conversions ultimately lead to high power at thedesired ROPA pump wavelength λ_(p) being present in the fiber travelingtoward the ROPA.

In a particular exemplary embodiment, a primary pump source at awavelength of 1276 nm is launched together with three lower-power seedsources having wavelengths of 1357, 1426 and 1480 nm. Energy at theprimary pump wavelength of 1276 nm first undergoes a stimulated Ramanconversion to 1357 nm and then, in the second step of a Raman cascade,the resulting high power at 1357 nm is converted to yield high power at1426 nm which in turn is converted to high power at the 1480-nm seedwavelength, the desired ROPA pump wavelength, through a further Ramanconversion.

In another exemplary embodiment, which is a variant of the foregoingexample, the seed source at 1357 nm is replaced by reflection means(e.g. a gold reflector or a fiber Bragg grating with a peak reflectivityat 1357 nm). Spontaneous Raman scattering of the high-power primary pumpat 1276 nm produces radiation in the 1357-nm region traveling in bothdirections in the fiber. As it travels in the fiber, this 1357-nmradiation is amplified due to the Raman gain at 1357 nm produced by the1276-nm pump. In addition, some of the outgoing 1357-nm radiationundergoes Rayleigh scattering and heads back toward the pump and seedlaunch terminal, being further amplified as it goes. The 1357-nmreflector sends the incoming amplified spontaneous Raman scatteredradiation back into the fiber, where it performs the same role as the1357-nm seed source in the foregoing example. In an extension of thisexemplary embodiment, the 1426-nm seed source is also replaced by areflector, thereby further reducing the number of active seed sources(e.g. laser diodes) required and further reducing costs.

In embodiments where a ROPA preamplifier is pumped along thetransmission fiber itself, the incoming signals experience distributedRaman amplification as they propagate from the ROPA towards thereceiving terminal due to the presence in the transmission fiber ofhigh-power at 1426 and 1480 nm and, for signals in the C-band, thepresent invention provides a much broader and flatter distributed Ramangain profile than the prior art, where the distributed Raman gain isprovided solely by high power at 1480 nm.

According to a broad aspect of the present invention, there is provideda system for increasing the amount of pump power that can be deliveredto a remote optically pumped amplifier in an optical fiber communicationsystem or, conversely, increasing the distance along the transmissionspan over which a given amount of pump power can be delivered, and whichcomprises: a primary pump source at wavelength λ₀, shorter than the ROPApump wavelength λ_(p); means to provide substantially lower energy attwo or more seed wavelengths λ_(s1) . . . λ_(sn), where n≧2 andλ₀<λ_(sn)≦λ_(p), and where the ensemble of seed sources contains atleast one at the ROPA pump wavelength λ_(p); and coupling means to inputenergy from the primary pump source and energy at the two or more seedwavelengths into said pump delivery fiber which may be either thetransmission fiber itself or a dedicated pump fiber; and wherein theprimary pump wavelength λ₀ is less than the wavelength λ_(p) by anamount corresponding to n Raman shifts in the delivery fiber and wherethe ensemble of seed wavelengths contains one in the vicinity of eachintermediate wavelength λ_(l), where l=n−1, n−2 . . . 1, and denotes thenumber of Raman shifts in the delivery fiber between the wavelengthλ_(l) and the ROPA pump wavelength λ_(p).

According to another aspect of the present invention, there is provideda system for increasing the amount of pump power that can be deliveredalong the transmission fiber to a remote optically pumped preamplifierand at the same time providing a broader, flatter distributed Raman gainprofile for the incoming signals as they propagate from the ROPA to thereceiving terminal, and which comprises: a primary pump source atwavelength λ₀, shorter than the ROPA pump wavelength λ_(p); means toprovide substantially lower energy at two or more seed wavelengthsλ_(s1) . . . λ_(sn), where n≧2 and λ₀<λ_(sn)≦λ_(p), and where theensemble of seed sources contains at least one at the ROPA pumpwavelength λ_(p); and coupling means at the receiving terminal to inputenergy from the primary pump source and energy at the seed wavelengthsinto the segment of the transmission fiber between the ROPA and thereceiving terminal; and wherein the primary pump wavelength λ₀ is lessthan the wavelength λ_(p) by an amount corresponding to n Raman shiftsin the delivery fiber and where the ensemble of seed wavelengthscontains one in the vicinity of each intermediate wavelength λ_(l),where l=n−1, n−2 . . . 1, and denotes the number of Raman shifts in thedelivery fiber between the wavelength λ_(l) and the ROPA pump wavelengthλ_(p); and wherein the seed wavelength λ_(sn) in the vicinity of λ_(l=1)is in the 1420-nm region.

According to a further broad aspect of the present invention, there isprovided a method for increasing the amount of pump power that can bedelivered to a remote optically pumped amplifier in an optical fibercommunication system or, conversely, increasing the distance along thetransmission span over which a given amount of pump power can bedelivered, the method comprising the steps of: providing a primary pumpsource at wavelength λ₀, shorter than the ROPA pump wavelength λ_(p);providing substantially lower energy at two or more seed wavelengthsλ_(s1) . . . λ_(sn), where n≧2 and λ₀<λ_(sn)≦λ_(p), and where theensemble of seed sources contains at least one at the ROPA pumpwavelength λ_(p); and propagating the energy at the primary pump andseed wavelengths in the pump delivery fiber which may be either thetransmission fiber itself or a dedicated pump fiber; and wherein theprimary pump wavelength λ₀ is less than the wavelength λ_(p) by anamount corresponding to n Raman shifts in the delivery fiber and wherethe ensemble of seed wavelengths contains one in the vicinity of eachintermediate wavelength λ_(l), where l=n−1, n−2 . . . 1, and denotes thenumber of Raman shifts in the delivery fiber between the wavelengthλ_(l) and the ROPA pump wavelength λ_(p).

According to another aspect of the present invention, there is provideda method for increasing the amount of pump power that can be deliveredalong the transmission fiber to a remote optically pumped preamplifierand at the same time providing a broader, flatter distributed Raman gainprofile for the incoming signals as they propagate from the ROPA to thereceiving terminal, the method comprising the steps of: providing aprimary pump source at wavelength λ₀, shorter than the ROPA pumpwavelength λ_(p); providing substantially lower energy at two or moreseed wavelengths λ_(s1) . . . λ_(sn), where n≧2 and λ₀<λ_(sn)≦λ_(p), andwhere the ensemble of seed sources contains at least one at the ROPApump wavelength λ_(p); and propagating the energy at the primary pumpand seed wavelengths toward the ROPA from the receiving terminal in thesegment of the transmission fiber between said ROPA and said receivingterminal; and wherein the primary pump wavelength λ₀ is less than thewavelength λ_(p) by an amount corresponding to n Raman shifts in thedelivery fiber and where the ensemble of seed wavelengths contains onein the vicinity of each intermediate wavelength λ_(l), where l=n−1, n−2. . . 1, and denotes the number of Raman shifts in the delivery fiberbetween the wavelength λ_(l) and the ROPA pump wavelength λ_(p); andwherein the seed wavelength λ_(sn) in the vicinity of λ_(l=1) is in the1420-nm region.

BRIEF DESCRIPTION OF THE DRAWINGS

The system will be better understood by an examination of the followingdescription, together with the accompanying drawings, in which:

FIG. 1 illustrates an exemplary system with a remote EDFA preamplifierand the prior art direct pumping scheme;

FIG. 2 illustrates the pump power evolution as a function of distancefrom the pump launch point for the prior art remote pumping scheme;

FIG. 3 shows the profile of the distributed Raman gain produced in ˜130km of PSCF when 1.3 W of 1480-nm pump power is launched into the fiber.

FIG. 4 is an exemplary system showing pumping of a remote EDFApreamplifier using a counter-propagating cascaded Raman pumping schemein the transmission fiber itself according to the present invention;

FIG. 5 illustrates a comparison of the pump power evolution as afunction of distance from the pump launch point for the prior art directremote EDFA pumping scheme and a counter-propagating cascaded Ramanpumping scheme according to the present invention;

FIG. 6 is an exemplary remote EDFA preamplifier with WDM couplersarranged to route the counter-propagating pump power arriving along thetransmission fiber to provide co-pumping of the EDFA to minimize theEDFA noise figure;

FIG. 7 is an exemplary system showing pumping of a remote EDFApreamplifier via the transmission fiber itself using acounter-propagating primary pump source, a seed source at the EDFA pumpwavelength and two seed ‘sources’ generated by wavelength-specific fiberBragg grating reflectors;

FIG. 8 is an exemplary system showing pumping of a remote EDFA postamplifier using a cascaded Raman pumping scheme over two dedicated pumpfibers according to the present invention.

FIG. 9 shows a comparison of the profile of the distributed Raman gainexperienced by the signals for pumping of a remote preamplifier alongthe transmission fiber for the prior art direct pumping scheme andthird-order cascaded Raman pumping according to the present invention;

FIG. 10 shows how the profile of the C-band distributed Raman gainexperienced by the signals for pumping of a remote preamplifier alongthe transmission fiber according to the present invention can beflattened by optimizing the choice of the intermediate seed wavelengthin the 1420-nm region and the EDFA pump seed wavelength near 1480 nm.

DETAILED DESCRIPTION

Remotely optically pumped amplifiers (ROPAs) provide a means ofincreasing the span lengths and/or capacities of unrepeatered fiberoptic communication links without requiring the placement of any activecomponents out in the span. This greatly simplifies the system,increases the overall reliability and reduces system cost. The maximumlink budget improvement possible through the inclusion of ROPAs isdirectly dependent on the maximum distance over which the required ROPApump power can be delivered. For ROPA post amplifiers, sufficient pumppower must be delivered to ensure that the saturated output power of theROPA is equal to the maximum composite signal power that can be launchedinto the transmission fiber without incurring transmission penalties dueto nonlinear effects in the fiber. For ROPA preamplifiers, sufficientpump power must be delivered to ensure that the preamplifier has a lownoise figure. The further the ROPAs can be placed from the pump launchterminals and still receive the required pump power, the greater theincrease in link budget resulting from their inclusion.

In the case of ROPA post amplifiers, the pump power is delivered via oneor more dedicated pump fibers (rather than the transmission fiber) toavoid Raman interactions between the pump and the signals leaving thetransmitter terminal. ROPA preamplifiers can be pumped along thetransmission fiber or along a dedicated pump fiber or a combination ofboth. When pumped along the transmission fiber, the ROPA pump alsoprovides distributed Raman preamplification for the incoming signals asthey propagate from the ROPA preamplifier towards the receivingterminal.

A generic prior art fiber optic telecommunication system employing aremotely pumped preamplifier is shown in FIG. 1. A long unrepeateredoptical fiber span is shown generally as 42, and the receiving terminalequipment generally as 40.

The long span fiber 42 consists of three component sections, a far fiber3, a near fiber 4, and a preamplifier erbium-doped fiber segment 5(typically several tens of meters in length), where the terms “near” and“far” represent distances relative to the receiving terminal 40. Thesignals 86 propagate from a distant transmitter or repeater terminal 1down fiber segment 3 to the input 6 of the ROPA preamplifier. The nowattenuated signals 44 are then amplified by the preamplifier and proceedfrom the ROPA output 7 on towards the receiving terminal 2 through fibersegment 4. In the prior art, the remote pump source 8 is typically ahigh power source at 1480 nm (e.g. a Raman fiber laser), the erbiumfiber pump wavelength. The pump output is launched into the transmissionfiber in a counter-propagating direction with respect to the signalsthrough the WDM coupler 9. As it propagates towards the ROPA, it isattenuated by the normal fiber attenuation, e.g. typically ˜0.2 dB/km inpure silica core fiber (PSCF). The high power at 1480 nm propagating infiber segment 4 provides distributed Raman amplification for the signalspropagating towards the receiving terminal 2.

In a pre-amplifier installation such as illustrated in FIG. 1, fiber 4is typically ≧100 km in length. Locating a pre-amplifier this far backin the span dramatically improves the equivalent Noise Figure (NF)compared to the case where the preamplifier is located at the receivingterminal since the preamplifier's ASE noise is now attenuated by thenormal fiber attenuation of fiber 4 before reaching the receiver.

FIG. 2 shows a graph of pump power evolution versus distance from thepump launch terminal 2 for the case of PSCF with an attenuation at 1480nm of 0.2 dB/km and a pump launch power of ˜1.3 W. The optimum locationof the remote preamplifier is a compromise between the desire to movethe amplification as far back in the span as possible and therequirement to deliver sufficient pump power to the erbium fiber toensure sufficient gain and a low noise figure. Typically, this requiresa delivered pump power of ˜8 to 10 dBm.

At first glance, it might appear that the solution is simply to increasethe pump launch power. However, there is a limit to the amount of pumppower that can be usefully launched. In PSCF, for example, if thelaunched pump power is increased beyond ˜1.3 W, the resulting high Ramangain (shown in FIG. 3) in the 1590-nm region begins to deplete the1480-nm power due to the build up of Raman ASE noise and ultimatelylasing oscillations near 1590 nm. Furthermore, for a remote preamplifierpumped along the transmission fiber, the distributed Raman gainexperienced by signals in portions of the C-band begins to becomedangerously high (i.e. greater than ˜30 dB) as the launched 1480-nmpower approaches 1.3 W and this can result in transmission penalties duemulti-path interference (MPI) arising from signal double Rayleighscattering. In fact, at a 1480-nm launch power of 1.3 W, the Raman gainover the upper 10 nm of the C-band exceeds this maximum limit, renderingthis region unusable for signal transmission.

FIG. 3 also illustrates that, when pumping along the transmission fiberaccording to the prior art, the distributed Raman gain experienced bysignals in the C-band varies by as much as 20 dB over the full C-band(1530 to 1565 nm) for a 1480-nm launch power of 1.3 W.

FIG. 4 shows a generic system in which the remote EDFA preamplifier 5 ispumped using a counter-propagating cascaded Raman pumping scheme in thetransmission fiber itself according to an exemplary embodiment of thepresent invention. In FIG. 1 and FIG. 4, like numerals refer to likeelements. The output of a high-power primary pump source 50 (typically aRaman fiber laser with an output power of several W) at wavelength λ₀ iscoupled into fiber section 16 via a WDM coupler 52 from where it passesthrough WDM couplers 62 and 10 and is launched into the transmissionfiber segment 4. The output of a low-power (typically <50 mW) EDFA pumpseed source 58 (typically a depolarized laser diode) at wavelength λ_(p)is coupled into fiber section 17 via WDM coupler 62 from where it passesthrough WDM coupler 10 and is launched into fiber segment 4. The outputsof first and second low-power (typically tens of mW) seed sources 12 and13 (typically depolarized laser diodes) at wavelengths λ_(s1) and λ_(s2)are combined via a WDM coupler 11, coupled into the transmission fibersegment 4 via WDM coupler 10 and launched to co-propagate with theenergy at the EDFA pump seed and primary pump wavelengths.

In the above exemplary embodiment, an essential element of the inventionis that the wavelength λ₀ of the high-power primary pump source isshorter than the EDFA pump wavelength λ_(p). More specifically, λ₀ mustbe less than λ_(p) by an amount corresponding to n successive Ramanshifts in the transmission fiber, where n≧2. It is also required thatthere be a source, among the seed source wavelengths, in the vicinity ofeach intermediate wavelength λ_(l), where l=n−1, n−2 . . . 1, anddenotes the number of Raman shifts in the fiber between the wavelengthλ_(l) and the EDFA pump wavelength λ_(p). In addition, it is requiredthat the ensemble of seed source wavelengths λ_(sn) includes the EDFApump wavelength λ_(p). For example, for an EDFA pump wavelength of 1480nm, λ₀, λ_(s1), λ_(s2) and λ_(s3) may be chosen as 1276, 1357, 1426 and1480 nm, respectively. In this example, n=3, λ_(s3) corresponds to theEDFA pump wavelength of 1480 nm, 1276 nm is less than this wavelength byan amount corresponding to three consecutive Raman shifts in silicafiber, the intermediate wavelength 1357 nm (λ_(s1)) corresponds to aRaman shift of 468 cm⁻¹ from 1276 nm, the second intermediate wavelength1426 nm (λ_(s2)) corresponds to a Raman shift of 357 cm⁻¹ from 1357 nmand the EDFA pump wavelength of 1480 nm (λ_(s3)) corresponds to a Ramanshift of 255 cm⁻¹ from 1426 nm. Although the peak Raman gain in silicafiber corresponds to a Raman shift of −440 cm⁻¹ (13.2 THz), the Ramangain profile is relatively broad (see FIG. 3), providing substantialgain even at Raman shifts significantly less than 440 cm⁻¹. In thiscase, the primary pump energy at 1276 nm provides distributed Raman gainfor the seed energy at 1357 nm, thus leading to high power at thiswavelength developing out in the transmission fiber. This high power at1357 nm in turn provides distributed Raman gain for the seed energy at1426 nm, ultimately leading to the presence of high power at thiswavelength even further out in the span and, finally, the high power at1426 nm provides distributed Raman gain for the 1480-nm seed energylaunched from the receiving terminal, resulting in high power at 1480 nmwhich reaches its peak value ˜25 km from the receiving terminal as seenin FIG. 5 where the evolution of the 1480-nm pump power in PSCF as afunction of distance from the receiving terminal is shown (curve 22) forthis exemplary case, with a 1276-nm launch power of 3.7 W, a pumpinglevel that is still below that which would lead to pump depletion due toexcessive Raman gain in the 1590-nm region.

For comparison, FIG. 5 also shows the pump power evolution in PSCF forthe prior art direct pumping scheme for the maximum allowable 1480-nmlaunch power of 1.3 W (curve 20). As can be seen, for pumping accordingto the present invention even at pumping levels less than the maximumtolerable, the peak value of the power at the EDFA pump wavelengthexceeds the maximum 1480-nm power reaching the 25-km point when pumpingaccording to the prior art. From that point on, the 1480-nm powerdeclines in both cases due to the fiber attention at 1480 nm (˜0.2dB/km), although the rate of decline is less for the present cascadedRaman pumping scheme due the continued presence of some Raman gain fromthe power at 1426 nm propagating in the fiber. The net result is thatpumping according to the present invention allows more 1480-nm pumppower to be delivered to the ROPA, in this case located 130 km from thereceiving terminal, than is possible with the prior art direct pumpingscheme.

As noted above, the Raman gain profile in silica fiber is relativelybroad, providing substantial Raman gain even at Raman shifts well below440 cm⁻¹. This offers a great deal of flexibility in selecting theoptimum intermediate wavelengths and, utilizing an intermediatewavelength in the 1420-nm region with either an intermediate seed (forn=3) or a primary pump (for n=2) in the 1355-nm region, makes itpossible to avoid having a pump or seed in the region of the broad‘water’ absorption peak of typical fibers at ˜1380 nm. This fact furtherenhances the pump delivery efficiency of the present invention since itavoids unwanted absorptive energy losses.

The choice of primary pump, ROPA pump and intermediate seed wavelengthsand powers, the number of primary pump, ROPA pump and intermediate seedwavelengths, the source of the energy at the primary pump, ROPA pump andintermediate seed wavelengths, the number, type and detailedarchitecture of the pump delivery fiber(s), the type of doped fibercomprising the ROPA, the detailed architecture of the ROPA, thewavelength band of the signals and the coupling means described in theabove exemplary embodiment are merely illustrative of the underlyingprinciple of the invention: namely, that the high power required at theROPA pump wavelength(s) in order to ensure delivery of adequate pumppower to the ROPA is developed and/or amplified and/or modified withinthe transmission fiber and/or dedicated pump fiber(s) through stimulatedRaman interactions initiated by launching energy at wavelength(s)shorter than the final ROPA pump wavelength(s) by an amountcorresponding to n successive Raman shifts, where n≧2.

For example, in a simplification of the above exemplary embodiment, aprimary pump wavelength λ₀ of 1357 nm and two seed sources at 1426 and1480 nm could be chosen. Conversely, a primary pump wavelength of 1090nm (readily available from Yb fiber lasers) and seed sources at 1145,1205, 1276, 1357, 1426 and 1480 nm could be chosen, yielding a six-stepRaman cascade (i.e. n=6). Furthermore, one or more dedicated pumpdelivery fibers could be added to the cable, with the ROPA being pumpedin a similar manner along both the transmission fiber and the dedicatedfiber(s) or only along the latter. The delivery fiber(s) may alsocontain segments of large effective area fiber or Raman ASE filters toreduce pump depletion due to Raman ASE build up. The ROPA architecturemay be designed to include WDM couplers 19 and 25, as shown in FIG. 6,arranged to route the counter-propagating pump power arriving along thetransmission fiber to provide co-pumping of the EDFA to minimize theEDFA noise figure. It will also be apparent to those skilled in the artthat the underlying principle of the invention is not limited toamplification of signals in the C-band nor to delivering pump powersolely to erbium-doped remote amplifiers.

FIG. 7 shows another exemplary embodiment in which the intermediate seedsources 12 and 13 of FIG. 4 are replaced by reflection means 14 and 15,exemplarily fiber Bragg gratings each with peak reflectivities at λ_(s1)and λ_(s2), respectively. In FIGS. 4 and 7, like numerals refer to likeelements. Referring to FIG. 7, as the high power primary pump energy atwavelength λ₀ propagates down the transmission fiber, it undergoesspontaneous Raman scattering, producing radiation with the classicRaman-shifted spectral profile traveling in both directions in thefiber. This spontaneous Raman scattered radiation is amplified as ittravels in the fiber due to the Raman gain provided by the high power atwavelength λ₀ present in the fiber. In addition, some of the outgoingspontaneous Raman scattered radiation undergoes backward Rayleighscattering and is further amplified as it travels back towards terminal2. Upon reaching terminal 2, that part of the amplified spontaneousRaman scattered radiation (denoted here as ASE by analogy to ‘amplifiedspontaneous emission’ in optical amplifiers) at λ_(s1) is reflected backinto the transmission fiber by reflector 14. In this exemplaryembodiment, an amplifying cavity for radiation at the seed wavelengthλ_(s1) is formed by the reflector 14 and a distributed Rayleigh‘mirror’. This leads to substantial energy at wavelength λ_(s1) beingpresent in the transmission fiber in the vicinity of terminal 2, whereit performs the same role as the launched seed source at λ_(s1) in theexample of FIG. 4 and, as discussed in connection with FIG. 4, thisleads to the build up of high power at λ_(s1) out in the span. The nextstep in the Raman cascade proceeds in exactly the same manner, with thehigh power at λ_(s1) providing Raman ASE in the region of λ_(s2) whichis similarly reflected from reflector 15, fulfilling the role of theseed source at λ_(s2) in the example of FIG. 4 and leading to the buildup of high power at λ_(s2) even further out in the span.

The exact details discussed in connection with FIG. 7 are not meant tolimit the principles of this embodiment of the invention: namely, thatthe energy at one or more of the seed wavelengths may be provided byutilizing reflection means in place of active sources (e.g. laserdiodes) at the seed wavelengths. For example, alternate reflection meanssuch as a gold reflector coupled to the transmission fiber through abroadband WDM coupler may be used in place of the fiber Bragg gratings.

FIG. 8 shows another exemplary embodiment in which a remote postamplifier 81 is pumped from the transmitter terminal along two dedicatedpump delivery fibers 32 and 36 incorporated in the fiber optic cable. InFIG. 8, 85 is the segment of the transmission fiber (typically ˜50 to 70km) leading out to the ROPA post amplifier, 86 is the follow-on segmentof the transmission span, 78 is an optical isolator and 77 is a lengthof erbium-doped fiber (typically several tens of meters). In thisexample, the ROPA pump assemblies feeding each of the pump fibers areidentical (although this is certainly not necessary), being comprised ofa primary pump source 30 (typically a high-power fiber laser), an EDFApump seed source 92 at wavelength λ_(p) (typically laser diode), a WDMcoupler 91 to combine the outputs of the primary pump and the EDFA pumpseed source and two fiber Bragg gratings 14 and 15 with peakreflectivities at λ_(s1) and λ_(s2), respectively. At the ROPA, the pumppower delivered by dedicated pump fiber 32 is coupled into the erbiumfiber in a co-propagating direction (with respect to the signals) via aWDM coupler 79 while the pump power delivered by the second dedicatedpump fiber 36 is directed into the erbium fiber in a counter-propagatingdirection via a WDM coupler 80. The dedicated pump fibers 32 and 36 maybe the same fiber type as the transmission fiber segment 85 or they maybe more complex in architecture. For example, they may incorporatesegments of large effective area fiber and/or ASE filters to reduce pumpdepletion due to Raman ASE build up. The follow-on transmission fibersegment 86 may advantageously consist of an initial segment of largeeffective area fiber to minimize nonlinear penalties due to the highsignal powers launched from the output of the ROPA.

The build up and delivery of the pump power at the ROPA pump wavelengthproceeds in the same way as that discussed in connection with FIG. 7.Dedicated pump delivery fibers are used when pumping a remote postamplifier to avoid Raman interactions with the signals traveling outalong fiber segment 85. The goal in the case of a remote post amplifieris not to provide distributed Raman gain for the signals, but rather todeliver sufficient pump power to the ROPA so that its saturated outputpower reaches the maximum tolerable signal launch power as determined bynonlinear transmission penalties. In this way, the effective location ofthe transmitter terminal is moved out in the span to the ROPA location.

The exact details discussed in connection with FIG. 8 are merelyillustrative and are not meant to limit the principles of thisembodiment of the invention. For example, two different EDFA pump seedwavelengths in the 1480-nm region could be selected to allow a portionof the pump power delivered by fiber 32 to be split off before reachingWDM coupler 79 and combined via a WDM coupler with the pump powerdelivered along fiber 36. This would allow the ratio ofcounter-propagating to co-propagating pump power to be optimized toincrease the saturated output power of the ROPA while at the same timeensuring a sufficiently-low noise figure.

FIGS. 9 and 10 illustrate a further aspect of the present invention.FIG. 9 shows a comparison of the profile of the distributed Raman gainexperienced by the signals for pumping of a remote preamplifier alongthe transmission fiber for the prior art direct pumping scheme as perFIG. 1 and third-order cascaded Raman pumping according to the presentinvention as shown in FIGS. 4 and 7. For the direct pumping scheme, theRaman gain varies by as much as 20 dB over the C-band, 1530 to 1565 nmand, from 1550 to 1565 nm, it exceeds 30 dB, which is approximately themaximum tolerable gain without multi-path interference (MPI)transmission penalties due to double Rayleigh signal scattering. Thislatter fact renders this portion of the C-band unusable for signaltransmission. In the case of third-order cascaded Raman pumping withintermediate seed wavelengths of 1357 and 1426 nm and an EDFA pump seedwavelength of 1480 nm, despite the fact that the pump power delivered tothe ROPA preamplifier is greater than that for the direct pumping scheme(see FIG. 5), the gain variation over the whole C-band is only ˜11 dBand, more importantly, the gain in the band only exceeds 30 dB in theregion below 1537 nm. Therefore, the gain is flatter and a greaterportion of the C-band is accessible for signal transmission.

FIG. 9 also shows that, for the cascaded pumping scheme, there is roomto increase the delivered pump power still further, by increasing the1276-nm launch power, since the Raman gain in the 1590 nm region isstill below the level at which pump depletion would occur due to thebuild up of Raman ASE and lasing oscillations; although, with the abovechoice of seed wavelengths, this would come at the expense of usablebandwidth for signal transmission, since the Raman gain would thenexceed 30 dB over a greater portion of the C-band. Confronting theseconflicting aims illustrates a further advantage of the presentinvention over prior art pumping schemes. Having an intermediate seedwavelength in the 1420-nm region, coupled with the breadth of the Ramangain in silica fiber and the relative insensitivity of the EDFA gain topump wavelength when pumping in the 1480-nm region, makes it possible tofine tune the profile of the gain across the C-band through thejudicious selection of the intermediate and EDFA pump seed wavelengths,thereby expanding the usable signal bandwidth for a given delivered ROPApump power. FIG. 10 shows the distributed Raman gain profiles for threedifferent selections of the intermediate and EDFA pump seed wavelengthsand for identical delivered ROPA pump powers. The gain below ˜1540 nm isprimarily due to the power at the intermediate seed wavelength presentin the fiber while, at longer wavelengths, it is largely due to thepower at the EDFA seed wavelength. As can be seen in FIG. 10, moving theintermediate seed wavelength down from 1426 to 1420 nm and the EDFA pumpseed wavelength from 1480 to 1485 nm substantially reduces the gain atboth ends of the C-band, thereby making the entire C-band usable forsignal transmission at this level of delivered ROPA pump power.Furthermore, this is accomplished without significantly increasing thepeak-to-peak gain variation across the C-band. Conversely, in systemswhere the signals occupy less than the full 35 nm of the C-band, thedelivered ROPA pump power could be increased still further without asevere penalty in usable signal bandwidth.

This flexibility in tailoring the profile of the Raman gain experiencedby the signals is a direct result of the cascaded Raman pumpingarchitecture of the present invention with an intermediate seedwavelength in the 1420-nm region.

EXAMPLE

An experimental fiber-optic transmission span, incorporating a ROPA postamplifier and a ROPA preamplifier, was setup. Sumitomo Z PSCF (loss at1550 nm=0.17 dB/km) was used throughout. The remote post amplifier waslocated 70 km from the transmitter terminal and was pumped along twodedicated pump fibers by two identical cascaded Raman pump modulessubstantially as shown in FIG. 8. The long-span fiber 88 carrying thesignals from the output of the post amplifier towards the ROPApreamplifier was 314 km in length. A variable optical attenuator wasinserted just before the remote preamplifier input to add an additional3 dB of span loss. The ROPA preamplifier architecture was substantiallyas shown in FIG. 6 and it was pumped from the receiving terminal alongthe transmission fiber by a cascaded Raman pump module substantially asshown in FIG. 7. The fiber segment 4 between the receiving terminal andthe ROPA preamplifier was 131 km long. The total span length wastherefore 70+314+131=515 km in length, yielding a total optical loss atthe C-band signal wavelengths of 87.5 dB. The added VOA loss (equivalentto 18 km of fiber) brought the total loss to 90.5 dB.

The three cascaded Raman pump modules each provided a primary pump powerof 3.7 W at 1276 nm, an EDFA pump seed power of 40 mW and included fiberBragg grating reflectors at intermediate seed wavelengths of 1357 and1426 nm. The ROPA preamplifier had a gain of 18 dB and a NF of 5 dB whenpumped by 5 mW of 1480-nm power. The use of the cascaded Raman pumpingscheme allowed both the ROPA post amplifier and preamplifier to beplaced ˜12 km further from their respective pump terminal than wouldhave been possible using the prior art direct pumping scheme, therebyincreasing the total span length by 24 km. Transmission over this linkof 4 STM16 (2.5 Gb/s) channels with wavelengths spanning the 1551.5 to1554-nm range was demonstrated with a BER of 10⁻¹⁰. With the addition offorward error correction (FEC), the total span loss budget forerror-free transmission increases to 94.5 dB, equivalent to a spanlength of 556 km.

1. A system to deliver pump power to a remote optically pumped amplifier(ROPA) in an optical fiber communication span, the span comprising asignal carrying fiber with the ROPA spliced into the signal carryingfiber at a distance from either the transmitter end forpost-amplification or from the receiver end for pre-amplification, andthe ROPA being pumped by energy at a pump wavelength λ_(p) carried by atleast one pump delivery fiber, said at least one pump delivery fiberhaving Raman properties whereby effective transmission of power at thepump wavelength λ_(p) is limited by a maximum launch power at λ_(p), thesystem comprising for each pump delivery fiber: a primary pump source atwavelength λ₀, shorter than the ROPA pump wavelength λ_(p); means toprovide substantially lower energy at two or more seed wavelengthsλ_(s1) . . . λ_(sn), where n≧2 and λ₀<λ_(sn)≦λ_(p), and where theensemble of seed wavelengths contains at least one at the ROPA pumpwavelength λ_(p); coupling means to input energy from the primary pumpsource and energy at the two or more seed wavelengths into said pumpdelivery fiber at said transmitter end for post-amplification or at saidreceiver end for pre-amplification, wherein the primary pump wavelength20 is less than the wavelength λ_(p) by an amount corresponding to nRaman shifts in the delivery fiber and where the ensemble of seedwavelengths contains one in the vicinity of each intermediate wavelengthλ_(l), where l=n−1, n−2 . . . 1, and denotes the number of Raman shiftsin the delivery fiber between the wavelength λ_(l) and the ROPA pumpwavelength λ_(p), said energy from the primary pump source and energy atthe two or more seed wavelengths coupled into said pump delivery fiberproviding more power at the pump wavelength λ_(p) to the ROPA than wouldbe provided by coupling the maximum input power level at the pumpwavelength λ_(p) into said pump delivery fiber at said transmitter orreceiver end.
 2. The system as claimed in claim 1, wherein said ROPA isa ROPA pre-amplifier and the said at least one pump delivery fiber isthe signal carrying fiber linking the ROPA and said receiver end.
 3. Thesystem as claimed in claim 1, wherein said ROPA is a ROPA pre-amplifierand the said at least one pump delivery fiber comprises the signalcarrying fiber linking the ROPA and said receiver end and one or morededicated pump delivery fibers.
 4. The system as claimed in claim 1,wherein said ROPA is a ROPA pre-amplifier and the said at least one pumpdelivery fiber comprises one or more dedicated pump delivery fibers. 5.The system as claimed in claim 2, wherein at least one of the λ_(s1) . .. λ_(sn) are selected to flatten the profile of the distributed Ramangain experienced by the signals due to the power at the λ_(s1) . . .λ_(sn) present in the signal carrying fiber.
 6. The system as claimed inclaim 1, wherein said ROPA is a ROPA pre-amplifier further comprisingmeans to couple light from the said at least one pump delivery fiberinto the ROPA amplifying fiber in a co-propagating direction withrespect to the signals.
 7. The system as claimed in claim 1, whereinsaid ROPA is a ROPA pre-amplifier, and said substantially lower energyprovided at one or more of said two or more seed wavelengths is providedby depolarized laser diodes.
 8. The system as claimed in claim 1,wherein the said ROPA is a post-amplifier and the said at least one pumpdelivery fiber comprises one or more dedicated pump delivery fibers. 9.The system as claimed in claim 8, further comprising means to couplelight into the ROPA amplifying fiber in both a co-propagating and acounter-propagating direction with respect to the signals.
 10. Thesystem as claimed in claim 9, wherein the ROPA is pumped by twodedicated pump fibers PF₁ and PF₂ and wherein the ROPA pump wavelengthin PF₁ is deliberately chosen to be different from, but closely spacedto, that in PF₂ and further comprising means to divide the ROPA pumpenergy delivered by PF₁ into two amounts of predetermined magnitude andmeans to combine one of said two amounts with the ROPA pump energydelivered by PF₂ prior to coupling the pump energy into the ROPAamplifying fiber, so as to optimize the ratio of co-propagating andcounter-propagating pump power coupled into the ROPA amplifying fiber.11. The system as claimed in claim 1, wherein said means to providesubstantially lower energy at said two or more seed wavelengths λ_(s1) .. . λ_(sn) includes reflection means to return into said pump deliveryfiber amplified spontaneous Raman scattered radiation, originating insaid pump delivery fiber due to the presence of high power at awavelength one Raman shift below the particular seed wavelength.
 12. Thesystem as claimed in claim 1, wherein the primary pump source is an Ybfiber laser operating at a wavelength in the 1090-nm region and thenumber of Raman shifts n between the primary pump wavelength λ₀ and thepump wavelength λ_(p) equals 6 and further comprising 5 fiber Bragggrating reflectors to provide the said substantially lower energy at theintermediate seed wavelengths λ_(si)≠λ_(p).
 13. The system as claimed inclaim 1, wherein the primary pump wavelength λ₀ and the number andposition of the intermediate seed wavelengths λ_(si)≠λ_(p) are chosenspecifically so as to avoid the water absorption peak in the pumpdelivery fiber.
 14. A method for pumping remote optically-pumped fiberamplifiers (ROPAs) in fiber-optic telecommunication systems, the spancomprising a signal carrying fiber with the ROPA spliced into the signalcarrying fiber at a distance from either the transmitter end forpost-amplification or from the receiver end for pre-amplification, andthe ROPA being pumped by energy at a pump wavelength λ_(p) carried by atleast one pump delivery fiber, said at least one pump delivery fiberhaving Raman properties whereby effective transmission of power at thepump wavelength λ_(p) is limited by a maximum launch power at λ_(p), themethod comprising: selecting primary pump wavelength λ₀, shorter thanthe ROPA pump wavelength λ_(p); selecting two or more seed wavelengthsλ_(s1) . . . λ_(sn), where n≧2 and λ₀<λ_(sn)≦λ_(p), and where theensemble of seed wavelengths contains at least one at the ROPA pumpwavelength λ_(p); coupling energy at the primary pump wavelength and atthe two or more seed wavelengths into said pump delivery fiber at saidtransmitter end for post-amplification or at said receiver end forpre-amplification, such that cascaded Raman amplification is used todeliver pump power to the ROPA that exceeds the pump power provided bycoupling the maximum input power level at the pump wavelength λ_(p) intosaid pump delivery fiber at said transmitter or receiver end.
 15. Themethod as claimed in claim 14, wherein said ROPA is a ROPA pre-amplifierand said at least one pump delivery fiber includes the signal carryingfiber, further comprising selecting at least one of the λ_(s1) . . .λ_(sn) to flatten the profile of the distributed Raman gain experiencedby the signals due to the power at the λ_(s1) . . . λ_(sn) present inthe signal carrying fiber.
 16. The method as claimed in claim 14,wherein said ROPA is a post-amplifier, further comprising incorporatingmeans to couple light into the ROPA amplifying fiber in both aco-propagating and a counter-propagating direction with respect to thesignals.
 17. The method as claimed in claim 16, wherein said ROPA ispumped by two dedicated pump fibers PF₁ and PF₂ and wherein the ROPApump wavelength in PF₁ is deliberately chosen to be different from, butclosely spaced to, that in PF₂ and further comprising incorporatingmeans to divide the ROPA pump energy delivered by PF₁ into two amountsof predetermined magnitude and means to combine one of said two amountswith the ROPA pump energy delivered by PF₂ prior to coupling the pumpenergy into the ROPA amplifying fiber, so as to optimize the ratio ofco-propagating and counter-propagating pump power coupled into the ROPAamplifying fiber.
 18. The method as claimed in claim 14, wherein saidcoupling of energy at said two or more seed wavelengths into said pumpdelivery fiber comprises using passive reflective means for couplingsaid energy at one or more of said two or more seed wavelengths.
 19. Themethod as claimed in claim 14, further comprising selecting an Yb fiberlaser operating at a wavelength in the 1090-nm region as the primarypump source and the number of Raman shifts n between the primary pumpwavelength λ₀ and the pump wavelength λ_(p) to equal 6 and furthercomprising using 5 fiber Bragg grating reflectors to couple said energyat the intermediate seed wavelengths λ_(si)≠λ_(p) into said pumpdelivery fiber.
 20. The method as claimed in claim 14, furthercomprising selecting the primary pump wavelength λ₀ and the number andposition of the intermediate seed wavelengths λ_(si)≠λ_(p) specificallyso as to avoid the water absorption peak in the pump delivery fiber. 21.The system as claimed in claim 3, wherein at least one of the λ_(s1) . .. λ_(sn) are selected to flatten the profile of the distributed Ramangain experienced by the signals due to the power at the λ_(s1) . . .λ_(sn) present in the signal carrying fiber.